1. Field of the Invention
The present invention relates to prosthetic heart valves and more particularly, to semi-biological heart valves consisting of mechanical structures together with parts made of biological material for optimal performance in vivo.
2. Prior Art
Prosthetic heart valves have been used for many years. The most common type of such valve incorporates a ball trapped in a cage. In recent years, other types of mechanical heart valves have been developed, which offer better flow characteristics than the ball in the cage type, but suffer from relatively high failure rates due to thrombus formation. This impairs the delicate motion of the various mechanical structures on which the functions of such valves depend. Some designs have a relatively high leak rate when closed; some produce high turbulence in the bloodstream; and nearly all cause damage to blood cells as a result of such cells being trapped and squeezed between the mechanical parts. As a result, nearly all patients using such valves are dependent on heparin.
Recently, valves incorporating purely biological materials have been successfully implanted, but the valves incorporating only biological material are very difficult to work with, and are subject to early failure when not installed in a particularly skillful manner.
Another promising design is a semibiological heart valve. Such a valve usually mimics the natural tricuspid heart valve and uses flexible biological material, formed either from real valve components or from other collageneous membranes. The structure of these valves is mechanical in nature, consisting of a metal or plastic ring, with posts, forming a platform to which the cusps are connected. The fabrication of such valves is very difficult, time consuming and expensive. The shape and position of all cusps must assure a good fit-tightness when closed, and the suture joints, by which the biological material is connected to the platform, must be very precise in order to provide a usefully long life for the valve.
The advantage of semi-biological valves is the gentleness with which they come to a closed position, so that few, if any, blood cells are damaged during valve closure. In addition, they are less noisy than purely mechanically arrangements.
However, the complicated shape is a disadvantage, not only in fabrication, but also in performance. If one cusp out of three stiffens up or deforms slightly, a minute thrombus formation gets lodged between the cusps and proper operation is thereafter impaired.
Although the semi-biological valves reduce the need for heparinization, which is crucially important, especially in the case of children, the small size valves required for children are the most difficult to be hand made because of their minute size and high complexity. The danger of thrombus formation is especially severe in the case of children, because thrombus retarding agents are usually contraindicated for children.
Recent studies have shown the deformation and stress of the collagenous materials of semi-biological valve components play an important role in their life expectancy before failure due to the delamination of the collagenous framework, producing cell infiltration and calcification. While the tension stresses in such a valve are well suited to the natural type of stress which collagen withstands, compression stresses, related often to bending of the cusps when opened, is extremely detrimental and most likely the primary cause for a series of a biological events ending in failure of such valves.
The natural valve has extremely thin leaflets in the cusps, making bending stresses very low. However, the prosthetic tricusp valve design is necessarily a compromise between strong and heavy leaflets for acceptable tension in the closed position, which are poor in bending stresses, or relatively thin ones which bend well but which are more inclined to fail in the suturing. The highly stress suture points at the corners of the cusps deliver most of the anchor forces, and the tricusp design offers no way out of this dilemma.